Improvements and fug bixes!

Improved (and renamed) loadFromString(), which is now called fromString() and kind of static. Just call Matrix.fromString() and you'll get a new instance. That's easy, right? Modified index.html to use Matrix.fromString().

The error messages when creating "wrong" matrices were pretty... confusing. I streamlined that. If you don't like it, don't merge it. :smile: 

Also fixed substraction that checked for wrong conditions.